This invention relates to the fabrication of optical waveguides and more particularly to optical fibers capable of single mode operation.
The majority of fibers presently provided are referred to as step-index multi-mode and graded index multi-mode fibers. A single mode fiber can propagate optical signals with low loss at extremely large bandwidths but are very difficult to fabricate.
Of the commercially available fibers, the step-index fiber is less expensive and basically consists of a glass core of uniform refractive index surrounded by a cladding glass of a slightly lower index of refraction. The more expensive graded index fiber has a core with a refractive index profile that is radially symmetrical and approximately parabolic in shape, being highest at the center of the core and decreasing parabolically until it matches the cladding refractive index at the core-clad interface.
In both step and graded index fibers, the light signal is carried in a large number of modes, each with a characteristic group velocity and propagation time. Single mode fibers are capable of accommodating larger bandwidths than either the step or graded fibers.
Using present day techniques, single mode fibers are still in the experimental stage due to the extremely small core sizes required. Certain techniques for providing single mode fibers require producing a core by the buildup of extremely thin layers. The layers are extremely difficult to control and hence, variations in composition degrade the fiber operation. As indicated, in order to accommodate single mode operation, one has to provide fibers with homogeneous cores having diameters of between 2 to 20 microns and this is extremely difficult using prior art techniques.
It is therefore an object to provide a fabrication process for producing single mode fibers having uniform core characteristics with extremely small diameters.